Question: Which of these numbers is composite? ${13,\ 15,\ 29,\ 61,\ 79}$
A composite number is a number that has more than two factors (including 1 and itself). 13, 29, 61, and 79 each have only two factors. The factors of 15 are 1, 3, 5, and 15. Thus, 15 is the composite number.